Method for driving liquid cell display panel

ABSTRACT

A method for driving a liquid cell in a scanning liquid cell display panel in which the liquid cell is interposed between two scanning electrode groups comprises the step of applying a DC voltage and an AC voltage having a frequency above the limiting frequency of the liquid crystal to said two scanning electrode groups during a period of semi-selection of the scanning while applying said DC voltage thereto during a period of full selection, an amplitude ratio of said AC to DC voltages during said period of semi-selection ranging from 1 to 2.

United States Patent Mitomo et al.

METHOD FOR DRIVING LIQUID CELL DISPLAY PANEL Inventors: lsamu Mitomo.Hachioji; Tetsunori Kaji; Masakazu Fukushima, both of Kokbunji. all ofJapan Assignee: Hitachi, Ltd., Japan Filed: July 20, 1973 Appl. No.:381,003

Foreign Application Priority Data July 21. 1972 Japan 47-73119 US. Cl..350/160 LC; 340/324 M; 340/166 EL Int. Cl. G02f 1/28 Field of Search350/160 LC; 340/324 M.

References Cited UNITED STATES PATENTS 4/1971 Nestor 350/160 LC h 1 t tt 1 1 June 24, 1975 Hucner ct al 350/161) LC X Torrcsi 350/161) LC XPrinmry E.t'uminerRonald L. Wibert Assistant Erumirwr-Paul K. GodwinAttorney, Agent, or Firm-Craig & Antonelli [57] ABSTRACT A method fordriving a liquid cell in a scanning liquid cell display panel in whichthe liquid cell is interposed between two scanning electrode groupscomprises the step of applying a DC voltage and an AC voltage hav ing afrequency above the limiting frequency of the liquid crystal to said twoscanning electrode groups during a period of semi-selection of thescanning while applying said DC voltage thereto during a period of fullselection, an amplitude ratio of said AC to DC voltages during saidperiod of semi-selection ranging from 1 to 2.

6 Claims, 11 Drawing Figures PATENTEI] JUN 24 I975 SHEET FIG. I

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PRIOR ART 1% 8 91. 3 O 6 PATENTEDJUN 24 I975 SHEET 2 FIG. 5

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0 2O 4O 6O 80 I00 I20 I40 (V) DC. PULSE METHOD FOR DRIVING LIQUID CELLDISPLAY PANEL BACKGROUND OF THE INVENTION 1. Field of the Invention Thepresent invention relates to a method for driving a liquid cell displaypanel. and more particulary to a method for driving a liquid celldisplay panel. in which a dynamic scattering mode produced during aperiod of semi-selection (hereinafter referred to as a semiselectionresponse) is controllable.

2. Description of the Prior Art In general. image or character displaydevices making use of a dynamic scattering effect of light in responseto a voltage applied to a liquid cell of the nematic type require thefunction of effecting selective determination of display positions. thatis. a scanning function. Upon application of a voltage to the liquidcell. there arises a big problem that an unpreferable dynamic scatteringmode usually called a crosstalk takes place during a period of s-calledsemi-selection.

SUMMARY OF THE INVENTION An object of the present invention is toprovide a driving method being capable of avoiding a semiselectionresponse.

Another object of the present invention is to provide a driving methodbeing capable of effecting display with good contrast.

In order to attain these objects. the present invention provides adriving method in which a high frequency voltage having a levelnecessary for cancelling the scattering mode is superimposed on a DCvoltage applied during the period of the semi-selection. and during aperiod of non-selection the high frequency voltage is made not to beapplied in order to prevent the falling time of the scattering mode frombeing reduced to an unpreferable extent.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic view showing aconventional liquid cell display panel.

FIG. 2 is a graph showing relative brightness ofa liquid cell uponapplication of a DC voltage.

FIGS. 3 and 4 are graphs showing conventional driving waveforms.

FIG. 5 is a graph showing relative brightness of a liquid cell uponapplication of a DC voltage with a high frequency voltage superimposedthereon.

FIGS. 6a and 6b are graphs illustrating an effect of the high frequencyvoltage upon the falling time of the dynamic scattering mode of a liquidcell.

FIGS. 7 to 9 are graphs showing embodiments of waveforms according tothe present invention.

FIG. 10 is a graph showing relative brightness ofa liquid cell uponapplication of a DC pulse voltage.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, there isschematically shown a liquid cell display panel ofthe nematic typeadapted for use in the present invention, which is a well-known displaypanel. In FIG. 1 a group of longitudinal lines (X,, X X,,,) arranged ina lateral direction represents a series of horizontal scanningelectrodes while a group of lateral lines (Y Y Y,,) represents a seriesof vertical scanning electrodes, both series of electrodes facing eachother through liquid cells. With such an arrangement. the liquid celllocated at an intersection P of the electrodes X and X effects a dynamicscattering mode upon the application of a predetermined voltage acrossthe electrodes X and Y In FIG. 2 there is shown a graph illustrating thedynamic scattering mode in terms of the voltage applied to the liquidcell in the above-mentioned liquid cell display panel. In FIG. 2 anabscissa represents the voltage applied to one electrode and measuredrelative to the other electrode serving as a reference while an ordinaterepresents relative scattering brightness B. Curves u and b showcharacteristics upon the application of the voltage having positive andnegative polarities. respectively. and marks V and V,,. show "athreshold voltage", that is. a voltage at which the dynamic scatteringmode of the liquid cell is initiated.

Next. a general driving method and a semi-selection mode will bedescribed in connection with FIG. 3. In FIG. 3 V shows a driving signalwaveform applied to the electrode X with a voltage V applied theretoduring the period of selection of I to I and V shows a driving signalwaveform applied to the electrode Y with a voltage applied theretoduring the periods of selection of r. to t and 1;, to 1 The distributionof the selection periods in FIG. 3 is arranged as a matter of convenience for the purpose of simplification of the drawings and thedescription. This applies to the following similar description anddrawings. In FIG. 3 there is shown a voltage (V,V between the desiredelectrodes X and Y at the intersection thereof to which the voltage 2Vis applied during the period of selection of t. to that is. during aperiod of selection to both the electrodes X and Y (hereinafter referredto as a period of full selection); the voltage V is applied during theperiods of selection of 1 to t and 1;, to t that is. during the periodof a so-called semi-selection in which one electrode is in the period ofselection and the other is in the period of no selection; and thevoltage zero is applied during the period of selection 1 to I that is.during the period of no selection relative to both the electrodes(hereinafter referred to as a period of non-selection). In this case, itis of importance that the dynamic scattering mode of the liquid cell isadapted to be effected only during a period when the period of selectionto both the electrodes coincides. that is. during the period of fullselection, and hence no dynamic scattering mode thereofis preferablyproduced during the periods of semi-selection and non-selection.

The reason is that the generation of the dynamic scattering mode duringthe period of semi-selection or nonselection causes a ratio of thecontrast of the dynamic scattering mode during the period of fullselection to that during the above-mentioned periods to be degraded.thus necessarily resulting in the indistinct display of the images orcharacters to be desired.

Accordingly, in order to prevent the dynamic scattering mode during theperiod of semi-selection, it is necessary to restrict the voltageapplied during the same period below the "threshold voltage" V shown inFIG. 2. The restriction of the applied voltage below V,,,. however.causes the voltage applied during the period of full selection to bebelow 2V because the voltage applied during the period of full selectionis twice as great as that during the period of semi-selection as shownin the waveform V of FIG. 3. Assuming that the threshold voltage isabout 8 volts, that level of the voltage becomes about In volts withreference to FIG. 2. This shows that the scattering brightness isrestricted down below a half maximum at this level of the voltage withthe result of no display of the satisfactory images or characters due tothe reduced brightness.

As an improvement of such a driving system there has been proposed aconventional driving system represented by the waveform of the voltagein FIG. 4.

The driving system of FIG. 4. has the same general driving method andthe same waveform of the voltage applied between both the electrodes asthat of FIG. 3 with the characteristic exception that a DC bias near tothe threshold voltage as shown in FIG. 2 is previously applied betweenboth the electrodes X and Y.

In this embodiment. the DC bias is applied to the electrode X with thevoltage V,,, applied during the period of no selection and with thevoltage V applied during the period of selection while the voltage zerois applied to the electrode Y during the period of no selection with thevoltage 2V,,, applied during the period of selection.

It will be appreciated from FIG. 4 that the voltage V between both theelectrodes is 3V,,, during the period of full selection and V,,, and Vduring the periods of semi-selection and non-selection, respectively.and therefore the voltage applied during the period of full selection isincreased at maximum three times as great as the threshold voltage.Accordingly. it will be calculated from FIG. 2 that the scatteringbrightness during the period of full selection reaches about 60 percentof its maximum. In any case, in the actual sanning system there is arequirement that the period of full selection should be as short aspossible; however, it is unpreferable to restrict the driving voltagebecause the scattering brightness relative to the level of the appliedvoltage is always reduced to the level lower than that shown in FIG. 2due to a relatively slow responce relative to the pulse voltage appliedto the liquid cell sometimes without any production of the dynamicscattering mode even upon the application of the voltage as high asseveral tens volts. At the present time, the improve ment of theresponse by the use of a high voltage drive is effective to theabove-mentioned problem, for the time being, other than the improvementof the liquid cell materials.

Prior to describing the present invention, two characteristics theliquid cell has will be described at first.

One of the characteristics is shown in FIG. 5 as a dynamic scatteringcharacteristic of the liquid cell upon the application thereto of the DCvoltage superimposed by the high frequency voltage. In FIG. 5 theabscissa indicates the high frequency voltage (the voltage beingrepresented by a peak value) while the ordinate indicates the relativescattering brightness. Bent curves 0. d and e show the relativescattering brightness relative to the high frequency voltagesuperimposed on the respective different DC voltages with the DC voltageincreased in the order of e, d and c. The same figure shows that theincrease in the high frequency voltage relative to each DC voltagecauses a reduction of the scattering brightness in more than a certainvalue thereof, eventually falling to zero at the voltages V V and V foreach bent lines 0, d and e.

Experiments carried out by the inventors reveal that a ratio of the highfrequency voltage to the DC voltage when the scattering brightness fallsdown to zero is about 2 in a normal state in the characteristic curvesshonn in Il(i. 5 while the ratio decreases more in a scanning statedepending on a time during the period of full selection. its cycle andthe level of the applied voltage with a lower limit reduced to about I.

The other characteristic is shown as an influence of the high frequencyvoltage upon the falling time in the scattering mode as shown in FIG.6a. in which a waveform g shows the waveform of the dynamic scatteringmode produced by the application of the driving voltage (6()\' 20milliseconds) having a waveform f with 50 percent attenuating time beingl5 milliseconds. FIG. 6a further shows that the 50 percent attenuatingtime of the waveform i of the light scattering mode is shortened to 2 to2.5 milliseconds when the liquid cell is drived by a waveform 11 furtherincluding a high frequency voltage (60V, 4KHz) superimposed on a por'tion ofthe waveformffollowing the period of selection.

On the other hand. a relation of a dynamic scattering amount (i.e..scattering brightness) to the frequency of the high frequency voltage isas shown in FIG. 6b (in which the abscissa shows the frequency F and theordinate shows the scattering brightness B) with a limiting frequency/bexisting therein. That is the scattering disappears above the limitingfrequency fiz The low frequency voltage (including the DC voltage)exists in the frequency region below fc while the high frequency voltageexists in the frequency region above fr'.

It will be. therefore. appreciated from the first characteristic (FIG.5) that it is possible to prevent the scattering mode if the highfrequency voltage at least greater than the voltage corresponding to V,.of FIG. 5 is superimposed on the DC voltage during the period ofsemi-selection. Further it will be understood from the secondcharacteristic (FIG. 6) that the application of the high frequencyvoltage during the period of nonselection causes the falling time of thescattering mode to be reduced.

This means that the high frequency voltage is not preferably to beapplied during the period of nonselection because it serves to cancelout a desirable memory effect.

In other words, the fundamental features of the present invention are,on the one hand, to superimpose on the applied DC voltage the highfrequency voltage equal to or greater than the voltage V,. shown in FIG.5 for cancelling the scattering mode during the period ofsemi-selection, and, on the other hand. to apply no high frequencyvoltage during the period of nonselection to prevent the falling time ofthe scattering mode from being shortened to an undesirable extent. It isto be noted that the DC voltage or low frequency voltage is primarilyapplied during the period of full selection as hithertofore to developthe scattering mode and that it is not hindered to superimpose the highfrequency voltage having as low amplitude as the scattering mode is notprevented.

Next, the driving voltages applied during each period of selection willbe described by way of embodiments.

A first embodiment is shown in FIG. 7 wherein numeral I indicates thewaveform V, of the driving signal applied to the electrode X (dottedlines hereinafter denoting the DC voltage) with the DC voltage of 1 voltsuperimposed by the high frequency voltage In of 2 volts during theperiod of selection of z, to 1 and with the voltage reduced to zeroduring the period of no selection of 1 to Numeral 2 indicates thewaveform V of the driving signal applied to the electrode Y [dottedlines hereinafter denoting the DC voltage) with the DC voltage of 2volts superimposed by the high frequency voltage of 2 volts during theperiods of selection of t, to and I;; to l and with the voltage of zeroapplied during the periods of no selection of 1 to I and t to 1 In thiscase. it is assumed that the high frequency voltage lu has the samefrequency. phase and voltage as the high fre quency voltage 2a. In thisfigure. numeral 3 shows the voltage waveform V, between the electrodes Xand Y at the intersection thereof to which the waveforms I and 2 areapplied. respectively. with the DC voltage of 3 volts applied during theperiod of full selection of r, to 1 with the DC voltage of 1 volt andthe high frequency voltage 312 of 2 volts applied during the period ofsemi-selection of to 1;, and the DC voltage of 2 volts and the highfrequency voltage 3c of 2 volts applied during the period of 1;, to rand with the voltage of zero volt applied during the period ofnon-selection. These values meet the above-mentioned fundamentalconditions of the present invention.

The above-mentioned embodiment shows that a ratio of the high frequencyvoltage to DC voltage becomes 2 at maximum during the period ofsemi-selection when the DC voltage is applied to the electrode X whilethe ratio thereof becomes 1 at maximum during the period ofsemi-selection when the DC voltage is applied to the electrode Y.

A second embodiment will be shown in FIG. 8 in which numeral 4 shows thewaveform V, of the driving signal applied to the electrode X with the DCvoltage of I volt applied during the period of selection of r, to 1;,and with the high frequency voltage 4b of 2 volts applied during theperiod of no selection of to Numeral 5 indicates the waveform V, of thedriving signal applied to the electrode Y with the DC voltage of 2 voltsapplied during the periods of selection of t, to 1 and I" to 1 and withthe high frequency voltage 5b of 2 volts applied during the periods ofno selection to I and I to 1 In this case. the high frequency voltage 4bis assumed to have the same frequency phase and voltage as the highfrequency voltage 5h. Further. in FIG. 8 numeral 6 indicates thewaveform V of the voltage between both the electrodes, which is the sameas the waveform 3 shown in FIG. 7.

It will be appreciated that although the different waveforms of thedriving voltage are applied to the electrodes X and Y in the first andsecond embodiments, the same scattering effect of the liquid cell isobtainablc.

A third embodiment shows a composite combination of the first typicalembodiment with the second typical embodiment with the waveform of thedriving signal shown in FIG. 9. In FIG. 9, numeral 7 indicates thewaveform V, of the driving signal applied to the electrode X with the DCvoltage of 4/3 volts and the high frequency voltage 7a of 2/3 voltsapplied during the period of selection oft to 1;, and with the highfrequency voltage 711 of2 volts applied during the period of noselection of to 15. The high frequency voltage 70 is herein assumed tohave the same frequency as the high frequency voltage 7h an 1 the phaseopposite thereto.

Further. in the same figure numeral 8 shows the waveform V of thedriving signal applied to the electrode Y with the DC voltage of 2 voltsapplied during the periods of selection of to 1 and 1;, to t and withthe high frequency voltage 8!) of 2 volts applied during the periods ofno selection of 1 to and I, to In this case, the high frequency voltage7b is assumed to have the same frequency. phase and voltage as the highfrequency voltage 8h.

Further. in FIG. 9 numeral 9 indicates the waveform V ofthe voltagebetween both the electrodes with the DC voltage of 3%; volts and thehigh frequency voltage 9a of 2/3 volts applied during the period of fullselection. with the DC voltage of l /is volts and the high frequencyvoltage 9b of 2% volts applied during the period of semi-selection of towith the DC voltage of 2 volts and the high frequency voltage 9c of 2volts applied during the period of selection of 1 to 1 and with thevoltage of zero volt applied during the period of non-selection.

An advantage of the third embodiment is that the DC voltage appliedduring the period of full selection is increased by I/3 volts althoughthe amplitude of the driving voltage ranges the same as that of thefirst and second embodiments. No problem arises with respect to the highfrequency voltage (9a in FIG. 9) subjected to the simultaneousapplication because it is low frequency voltage of 2/3 volts.

The use of the driving system as mentioned above causes no dynamicscattering mode during the period of semi-selection. thus permitting animprovement of an optical contrast.

That is. in FIG. I0 there is shown a relation of the applied voltage tothe relative brightness for each duty ratio of one-tenth to one-fiftiethwhen the DC pulse voltage is applied to the liquid cell. Accordingly,with the conventional driving waveform as shown, for example, by V, inFIG. 4 a maximum driving voltage is 3 X 8 (volts) 24 (volts) when thethreshold voltage is set to be 8 volts. but the maximum driving voltageexperimentally reaches 36 volts in a scanning state (in the duty ratioof one-tenth) because the threshold voltage increases more than 8 volts.As a result, the relative brightness becomes 0.36 from thecharacteristic curve with the contrast where 0.06 represents backgroundbrightness.

In contrast thereto, according to the present invention in which nodynamic scattering mode occurs during the period of semi-selection. thelevel of the driving voltage is experimentally as high as I00 voltstaking into account the break-down voltage or life span of the liquidcell. so that the relative brightness is 1.5 with the contrast As aresult, the present invention permits the contrast to be improved aboutfour times as much as that of the conventional one.

In the description above mentioned, a ratio of the high frequencyvoltage to the DC voltage applied during the period of semi-selectionhas been selected to be I and 2. but suitably ranges from 1 to 2depending on the characteristics of FIG. 5. Further. it will beappreciated that the driving waveform is not limited to the aboveembodiments but only required to be of alternating waveform.

We claim:

I. A method for driving a liquid cell display panel including aplurality of first electrodes arranged in parallel to each other, aplurality of second electrodes re spectively intersecting with each ofsaid first electrodes and arranged in parallel to each other, and liquidcells of nematic type disposed at the respective intersections of saidfirst electrodes with said second electrodes, said method comprising thestep of applying a voltage to a predetermined liquid cell of said liquidcells through said first and second electrodes to drive saidpredetermined liquid cell, wherein said voltage includes a DC voltageand a high frequency voltage superimposed thereon during periods of twotypes of semi-selection of said liquid cell and said voltage includes atleast a DC voltage during a period of full selection thereof, anamplitude ratio of said high frequency voltage to DC voltage during theperiod of at least one of said two types of semi-selection being in therange of at least 1 and not more than 2.

2. A driving method as set forth in claim 1, wherein a low frequencyvoltage is substituted for said DC volt age.

3. A driving method as set forth in claim 1 including applying a DCvoltage having a high frequency voltage superimposed thereon to a firstelectrode or a second electrode associated with a respective liquid cellduring the period of semi-selection of said liquid cell.

4. A driving method as set forth in claim 1, wherein the DC voltage andthe high frequency voltage are applied during the entire period ofsemi-selection so as to provide said amplitude ratio.

5. A driving method as set forth in claim 1, wherein the amplitude ratioof the high frequency voltage to the DC voltage is 2.

6. A method as set forth in claim 1, wherein the amplitude ratio of highfrequency voltage to DC voltage is in the range of more than 1 and notmore than 2.

1. A method for driving a liquid cell display panel including aplurality of first electrodes arranged in parallel to each other, aplurality of second electrodes respectively intersecting with each ofsaid first electrodes and arranged in parallel to each other, and liquidcells of nematic type disposed at the respective intersections of saidfirst electrodes with said second electrodes, said method comprising thestep of applying a voltage to a predetermined liquid cell of said liquidcells through said first and second electrodes to drive saidpredetermined liquid cell, wherein said voltage includes a DC voltageand a high frequency voltage superimposed thereon during periods of twotypes of semi-selection of said liquid cell and said voltage includes atleast a DC voltage during a period of full selection thereof, anamplitude ratio of said high frequency voltage to DC voltage during theperiod of at least one of said two types of semi-selection being in therange of at least 1 and not more than
 2. 2. A driving method as setforth in claim 1, wherein a low frequency voltage is substituted forsaid DC voltage.
 3. A driving method as set forth in claim 1 includingapplying a DC voltage having a high frequency voltage superimposedthereon to a first electrode or a second electrode associated with arespective liquid cell during the period of semi-selection of saidliquid cell.
 4. A driving method as set forth in claim 1, wherein the DCvoltage and the high frequency voltage are applied during the entireperiod of semi-selection so as to provide said amplitude ratio.
 5. Adriving method as set forth in claim 1, wherein the amplitude ratio ofthe high frequency voltage to the DC voltage is
 2. 6. A method as setforth in claim 1, wherein the amplitude ratio of high frequency voltageto DC voltage is in the range of more than 1 and not more than 2.